The need to shield electronic components from electromagnetic interference ("EMI") and radio frequency interference ("RFI") is well known. While traditional metal cabinets provide an effective shield against this interference, plastic housings permit EMI and RFI waves to pass through. Sensitive electrical components must therefore be housed in metal housings or the plastic housing must be coated with a thin layer of metal to shield against interference.
A variety of methods are known for coating a thin layer of metal onto plastics to make them shield against EMI and/or RFI. One such method is plastic plating, although that method has proven to be expensive, environmentally undesirable, and inconsistent. Vacuum metallizing is another alternative, but that method is unsuitable for volume production of electronic enclosures. The incorporation of metallic fiber into the plastic itself is another method with some promise, but that method remains experimental and expensive and unavailable for widespread use.
The most preferred method of coating metal onto plastic is by spray painting. In particular, spray paints with metal particles (including, e.g., metal powders and flakes) suspended in the paint may be advantageously used. The metal particles are commonly nickel or copper, although silver or some other electrically conductive metal may be used.
Nickel-based EMI-shielding and RFI-shielding coatings were the first to be developed. Although those coatings are relatively inexpensive to make and use, their use has diminished in recent years due to environmental concerns regarding both the nickel itself and the solvents which must be used in those compositions.
Copper-based EMI-shielding and RFI-shielding coatings have been developed to address some of the problems associated with nickel-based coatings. Copper-based coatings are somewhat more expensive than nickel-based coatings, but higher conductivity may be obtained therewith. Either nickel- or copper-based coatings may be prepared with an aqueous carrier so that a low volatile organic content (VOC) may be obtained and certain environmental considerations may be mitigated. However, known copper-based coatings require caustic strippers, are very unstable (they oxidize in the can), and typically provide a poor suspension and poor color flow, etc.
A need therefore exists for an aqueous-based EMI-shielding and RFI-shielding coating composition with good strippability, stability, suspension, color flow, etc. The present invention addresses that need.